He praised Thomas for his faith, saying to him, “Because you have seen Me, have you believed?” But looking ahead to the time when the tangible, physical evidence Thomas had witnessed would no longer be available, the Lord pronounced those “blessed … who did not see, and yet believed” (cf. 2 Cor. 5:7; 1 Peter 1:8–9). They, who will never see physical evidence of Christ’s rising, will have a greater measure of the Holy Spirit to empower faith in the resurrection. This is the second beatitude in this gospel (cf. 13:17). Blessed does not just convey a condition of happiness, but also declares the recipient to be accepted by God.
It must be noted that our Lord’s words do not indicate anything defective about the faith of Thomas.
Thomas’s faith is not depreciated … “but for the fact that Thomas and the other apostles saw the incarnate Christ there would have been no Christian faith at all. Cf. 1:18, 50f.; 2:11; 4:45; 6:2; 9:37; 14:7, 9; 19:35” (Barrett, p. 573).… later believers come to faith through the word of the earlier believers (17:20). Blessed, then, are those who cannot share Thomas’ experience of sight, but who, in part because they read of Thomas’ experience, come to share Thomas’ faith. (D. A. Carson, The Gospel According to John, The Pillar New Testament Commentary (Grand Rapids: Eerdmans, 1991), 660)
MacArthur New Testament Commentary